This invention relates to a type of injection laser and monitor photosensor assembly in which a reflector is employed to direct light emanating from a facet of the laser onto the photosensitive zone of the photosensor for monitoring the performance of the laser. It finds particular application in such types of assembly in which the laser is operated at high speed and the photosensor is required to respond to this modulation, which may for instance be at a frequency in excess of 2 GHz, typically in the region of 2.5 GHz. Typically a photosensor able to respond to such frequencies has a relatively small photosensitive surface, and so the practice, used for slow speed monitor photodiodes, of locating the photodiode at some distance behind the laser to intercept unguided light, is inappropriate for the high speed application because it would produce an inconveniently small photo current.
One solution to this problem would be to mount the high speed photosensor much closer to the laser facet, so that its photosensor surface intercepts much more of the divergent emission from the rear facet of the laser. A primary drawback of this approach is that close proximity is liable to introduce problems of excessive electrical cross-talk between the laser drive circuitry and the photosensor circuitry. An alternative solution is to keep the photosensor at some distance from the laser facet, and to introduce some form of focusing system between the two integers, for instance a lens or mirror. It is already known, for instance from GB-A-2 229 856, to use a microlens to couple light from an injection laser into the end of an optical fibre, and in principle the same sort of microlens arrangement can be used to couple light from the other end facet of the laser into a monitor photosensor. In practice however this optical coupling of the monitor photosensor to the laser is more difficult to implement. In the alignment of the fibre, the associated microlens can be secured in substantially the correct position, and then the fibre end can similarly be secured in the position providing substantial optimisation of the optical coupling, as determined by powering the laser and observing the amount of optical power to emerge from the far end of the store. Thereafter, fine adjustment of the positioning of the intervening microlens may be required to complete the optimisation. No particular difficulty is encountered in continuously observing the optical power output from the far end of the fibre all the time the position of its other end is being adjusted. On the other hand, to observe the output of the monitor photosensor, while its position is being adjusted, involves a requirement for electrical connection with that photosensor to be established before it has been fixed down in position, and therefore calls for the provision of flying leads or their equivalent.